Out-of-band blocking, OOBB, is a measure of the ability of a radio device to suppress strong interferer signals outside the receive band of the device. Most radio performance requirements and specifications include OOBB requirements. Verification of these requirements is a time consuming and therefore costly process.
For a radio receiver supporting duplex communications, the selectivity (adjacent channel, in-band and out-of-band blocking) of the receive path is tested with the own transmitter at high power. The receiver path must be able to:                1. reject a strong external interferer outside the receive frequency band without receiver saturation, and        2. reject the said external interferer in the presence of another strong signal (the own transmitter) at a frequency different from the external interferer.        
Firstly, the receiver may be saturated (blocked) or suffer degraded performance in the presence of a strong interferer. Secondly, intermodulation between the external interferer and other signals internal to the radio device under test, e.g. the own transmitter, may create intermodulation products that fall within the receiver channel. The intermodulation is caused by nonlinearities in the receiver (or transceiver).
FIGS. 1 and 2 depict two interferer scenarios for out-of-band blocking tests. FIG. 1 shows the external interferer at a certain frequency separation, Fseparation, from the receive channel (cross-hatched area) assigned in a certain operating band (un-hatched area). The receiver must be able to withstand a larger interferer power as the frequency separation to the receive channel increases; a measure of the receiver selectivity.
FIG. 2 shows a scenario in which the external interferer and the own transmitter (striped area) generate an intermodulation product (solid black area) that falls within the receiver channel. Then a spurious response is generated. As the skilled person will realize, a spurious response is a measure of the receiver's ability to receive a wanted signal on its assigned channel frequency without exceeding a given degradation due to the presence of an unwanted continuous wave, CW, interfering signal at any other frequency at which a response is obtained, i.e. for which a specified out of band blocking limit is not met.
In this case, this occurs when the separation between the external interferer and the own transmitter is equal to the frequency separation between the own transmitter and receiver: FDuplex (a so called double-duplex interferer). The magnitude of the intermodulation product generating the spurious response depends on the power levels of the external interference and the own transmitter.
An out-of-band blocking (selectivity) test is carried out with the own transmitter at a specified power, typically close to the maximum output power of the radio device tested, and a given wanted signal level close to the minimum power level of the receiver. This means that a larger external interferer power can be tolerated when (a) spurious responses are not generated and when (b) the frequency separation between the external interferer and the receive channel increases. FIG. 3 shows a case where a spurious response is not generated: the receiver can then tolerate a higher external interference power since the wanted signal is not degraded by the intermodulation product.
Spurious responses can also be generated by intermodulation with other internal signals such as harmonics of the local oscillator. During a selectivity test, it is important that the external interference is swept in frequency over a large range, typically from the order of 20 kHz to 10 GHz for commercial radio equipment. The spurious responses generated by various internal signals should be covered. In practice the interferer is stepped in frequency to cover the frequency range, and the test time required is essentially inversely proportional to the step size.
The test time due to capture spurious response and a large frequency range is very long given typical step sizes used. This is exacerbated if the radio device supports features like carrier aggregation (for the air interface of universal terrestrial radio access, UTRA, systems and evolved universal terrestrial radio access, E-UTRA, systems) when several combinations of transmitter- and receive channels must be tested (performed in sequence). For example, when testing E-UTRA (long term evolution, LTE) radio equipment, the step size currently is 1 MHz and the out-of-band blocking tests requires around 8 hours for completion.